Author
Topic: Landing on the Moon using a tail hook? (Read 13345 times)

Not on the first mission or the second, but if a lunar colony were established, what would be the potential of preparing a very long lunar landing strip with arrestor cables to decelerate lunar landers equipped with a tail hook? The analogy of aircraft carrier landings comes to mind.

As I envision it the lander, equipped with landing skids or wheels, would approach the landing strip in a Moon grazing orbit with landing engines oriented downward (horizontal lander) to avoid slamming into the surface when the tail hook engaged the arrester cable. The cable would be very long and the lander would skid to a stop before the end of the runway. Maybe 2-3 g's deceleration? How long would the strip need to be and would it be possible to do a wave off and go around? What technology would be needed before such a technique could be considered? Are there long enough, smooth enough, high enough places on the moon that such a landing strip could be built and what delta V savings might be achievable?

The issue is setting up the "glide path". Since there is no air, it would be an orbit that is tangential to the "landing strip. So you want to go close to the moon going at more than 3500 mph?

Well, by the time a colony exists, the lunar gravity field should be well mapped so how much more dangerous is approaching within tail hook distance to the surface in a 3500 mph orbit than approaching tail hook distance at 350 mph glide slope to an aircraft carrier. Both are fatal if you're to low.

I would allow that we might want a little longer tail hook for use on the moon.

Fighter jets are doing a lot less than 350 mph when they land ... I did a quick google, and 130 knots / 150 mph is closer to the actual landing speed ...

Energy goes up as velocity squared, so you're looking at 20+ times the velocity, so 400+ times the energy.

3500 mph is 1565 m/sec. If you decided that you'd accept a 10G deceleration (98 m/sec^2), you'd take 16 seconds to decelerate to zero. Your average speed is 780 m/sec, so you'd travel about 12.5 km before stopping. At a more reasonable 5 Gs, you're looking at 32 seconds and 25 km to stop.

That's a LONG arresting cable .... (and we haven't even looked at the forces on the cable, which needs to go almostinstantaneously from rest to 1565 m/sec without snapping ....)

Well, by the time a colony exists, the lunar gravity field should be well mapped so how much more dangerous is approaching within tail hook distance to the surface in a 3500 mph orbit

wrong, gravity field mapping is a minor/secondary effect. The issue is precision of the "deorbit" burn that sets up the orbit "glideslope" that intersects with the landing field. It can't be done accurately enough to ensure alignment vertically, laterally and short/long. The corrections for the errors end up being orbital corrections in reality. Since it is a low point in the orbit, the corrections are at the worst spot in terms of effectivity. Laterally adjustments are plane changes, long/short adjustments are orbit axis changes. These will be huge in terms of propellant. This makes the concept highly unlikely that it is viable. And there still is the risks flying low that fast.

Well, by the time a colony exists, the lunar gravity field should be well mapped so how much more dangerous is approaching within tail hook distance to the surface in a 3500 mph orbit

wrong, gravity field mapping is a minor/secondary effect. The issue is precision of the "deorbit" burn that sets up the orbit "glideslope" that intersects with the landing field. It can't be done accurately enough to ensure alignment vertically, laterally and short/long. The corrections for the errors end up being orbital corrections in reality. Since it is a low point in the orbit, the corrections are at the worst spot in terms of effectivity. Laterally adjustments are plane changes, long/short adjustments are orbit axis changes. These will be huge in terms of propellant. This makes the concept highly unlikely that it is viable. And there still is the risks flying low that fast.

Certainly valid technical problems to be solved.

The "short" landing problem can be solved using the vertical lift thrusters given sufficiently advanced knowledge of the distance short. The "long" problem looks like a "go around" to me. Isn't vertically just another version of short and long? As for lateral, how much lateral velocity do you suppose the tail hook-arrester cable can accommodate?

I did a little calculation and found that a 5.5 mile (29,000 feet) landing strip would need 14 g's deceleration and since distance is inversely proportional to deceleration for a fixed initial velocity, at 7 g's decal, landing strip length is doubled to 11 miles. Same thing with time, decelerate at 14 g's for 11.4 seconds, or 7 g's for 23 seconds. 3.5 g's takes 46 seconds and 22 miles. Pretty rugged cargo and lander that.

Funny (and therefore interesting, at least for a an arm-chair rocket scientists like me) idea.

If a mass driver exists on the Moon, the numbers involved (acceleration, energy, accuracy requirements, etc.) are more or less the same.

At this point I guess that a mass driver, or a hook mounted on a mass driver, could be a better solution.

I don't mean the mass driver is easier or doable. Just remarking that a tail hook is the same class of approach to the problem of accelerating/decelerating in vacuum, except unwinding a cable several km long, rather than using a rail and magnetic forces or friction to land at that speed means probably gigantic wheels and h/w involved.

Don't forget that as a craft comes down, it starts to form an arc with it's path as it loses speed. To maintain a steady spiral, you'd actually have to add velocity to compensate for the gravitic drag.

Actually, you'd most likely be better off with a sort of magnetic catch net to decelerate the craft as it came down vertically. No actual net, but magnetic fields to reduce the speed of descent.

Don't forget that as a craft comes down, it starts to form an arc with it's path as it loses speed. To maintain a steady spiral, you'd actually have to add velocity to compensate for the gravitic drag.

What do you mean by "spiral?" The lander inserts into a near circular elliptical orbit with the perilune at and aligned with the landing strip. It does not loose speed until it snags the arrestor cable. It gains a little speed as it looses altitude around the orbit.

The "short" landing problem can be solved using the vertical lift thrusters given sufficiently advanced knowledge of the distance short. The "long" problem looks like a "go around" to me. Isn't vertically just another version of short and long? As for lateral, how much lateral velocity do you suppose the tail hook-arrester cable can accommodate?.

And what size are the vertical lift thrusters and how much propellant is needed.

A tail hook-arrester cable can accommodate no lateral velocity if the vehicle is not over the strip laterally

Don't forget that as a craft comes down, it starts to form an arc with it's path as it loses speed. To maintain a steady spiral, you'd actually have to add velocity to compensate for the gravitic drag.

What do you mean by "spiral?" The lander inserts into a near circular elliptical orbit with the perilune at and aligned with the landing strip. It does not loose speed until it snags the arrestor cable. It gains a little speed as it looses altitude around the orbit.

Basic aeronautics; Gravity is a downward force that also acts as a form of drag.

Even without an atmosphere, velocity will be lost due to the lunar gravity. The arc would tend to be like that of a cannon ball. While the cannonball will go farther, gravity will still pull it down in an arc. the speed that it gains is in a downward fashion while forward momentum is bled off by gravity.

Don't forget that as a craft comes down, it starts to form an arc with it's path as it loses speed. To maintain a steady spiral, you'd actually have to add velocity to compensate for the gravitic drag.

What do you mean by "spiral?" The lander inserts into a near circular elliptical orbit with the perilune at and aligned with the landing strip. It does not loose speed until it snags the arrestor cable. It gains a little speed as it looses altitude around the orbit.

Basic aeronautics; Gravity is a downward force that also acts as a form of drag.

Even without an atmosphere, velocity will be lost due to the lunar gravity. The arc would tend to be like that of a cannon ball. While the cannonball will go farther, gravity will still pull it down in an arc. the speed that it gains is in a downward fashion while forward momentum is bled off by gravity.

I don't think that's right. If it were right then to maintain LEO would require continuous boosting beyond boosting to counter aerodynamic drag and it does not.

Even without an atmosphere, velocity will be lost due to the lunar gravity. The arc would tend to be like that of a cannon ball. While the cannonball will go farther, gravity will still pull it down in an arc. the speed that it gains is in a downward fashion while forward momentum is bled off by gravity.

As long as the spacecraft has not other forces acting upon it, it's in orbit, and therefore subject to the laws of gravity only. You may imagine an orbit tangential to the surface at the point it must be hooked. Than you need some adjustments. But there is not necessarily any ballistic arc involved like you mentioned. Everything depends on the regular trajectories laws.

You may figure it out by temporally inverting a mass driver launch under vacuum: at the point the spacecraft reaches orbital speed, some longitudinal thrust may make it lift. No vertical thrust necessarily needed.

You temporally invert this and you get exactly what you can and you can't do.

The point is there is no ned to solve them. There is no benefit to the idea.

I estimate benefit using Isp = 400 seconds for the lunar vehicles. The benefit is the savings of a half tonne of prop in low lunar orbit for every tonne of lander + payload set down on the moon. Prop in low lunar orbit is expensive as it takes another half tonne of prop to lift every tonne of tanker + prop to low lunar orbit. Unless the prop comes from the Earth which is very much more expensive, but I'm suggesting an existing lunar colony.

The "short" landing problem can be solved using the vertical lift thrusters given sufficiently advanced knowledge of the distance short. The "long" problem looks like a "go around" to me. Isn't vertically just another version of short and long? As for lateral, how much lateral velocity do you suppose the tail hook-arrester cable can accommodate?.

And what size are the vertical lift thrusters and how much propellant is needed.

A tail hook-arrester cable can accommodate no lateral velocity if the vehicle is not over the strip laterally

Well obviously the vertical lift thrusters on the "Eagle" were enough for the mass of the descending "Eagle." That was Apollo 11 wasn't it?

And, as I recall the Eagle landed long, but I don't know how far laterally the trajectory was from the target landing area. If the lander trajectory is laterally displaced from the landing strip that is another case where a "go around" is a solution. It would require a new landing trajectory calculation but calculations are cheap.

Basic aeronautics; Gravity is a downward force that also acts as a form of drag.

If you mean "gravity drag" as for rocket launch from the Earth, this is not necessarily true.

On Earth we launch vertically because we have a dense atmosphere. Therefore, we can't accelerate like an aircraft at more than 1-2 Mach without having to gain high altitude before getting from 2 to Mach 25.

But there is actually no reason to launch vertically in vacuum if you have a veeeeeery long runway (no, Apollo LMs haven't...). That's the point of having a mass driver. In this case there is NO gravity drag, because the gravity drag - at least this is what I understood - is just a way to take in account the fact that you have to lift the fuel (i.e. not only accelerate it) on top of the dry mass of the rocket when you have a vertical component of the speed at launch.

Hooking a cable across a runway is ridiculous, like trying to catch a APDS tank round, it'd cut through any solid object. You'd have to either use magnetic fields or a gas, and I've no idea how a gas is supposed to be contained on a runway in a vacuum..

Logged

When my information changes, I alter my conclusions. What do you do, sir?John Maynard Keynes